Large area semiconductive organic-based devices for producing light from electrical energy (lighting sources) and devices for producing electrical energy from light (photovoltaic sources) may be used in a wide variety of applications. For instance, high efficiency lighting sources are continually being developed to compete with traditional area lighting sources, such as fluorescent lighting. While light emitting diodes have traditionally been implemented for indicator lighting and numerical displays, advances in light emitting diode technology have fueled interest in using such technology for area lighting. Light Emitting Diodes (LEDs) and Organic Light Emitting Diodes (OLEDs) are solid-state semiconductor devices that convert electrical energy into light. While LEDs implement inorganic semiconductor layers to convert electrical energy into light, OLEDs implement organic semiconductor layers to convert electrical energy into light. Generally, OLEDs are fabricated by disposing multiple layers of organic thin films between two conductors or electrodes. The electrode layers and the organic layers are generally disposed between two substrates, such as glass substrates. When electrical current is applied to the electrodes, light is produced. Unlike traditional LEDs, OLEDs can be processed using low cost, large area thin film deposition processes. OLED technology lends itself to the creation of ultra-thin lighting displays. Significant developments have been made in providing general area lighting implementing OLEDs.
However, while traditional OLEDs having a relatively low efficacy (e.g. 3–4 lumens per watt) may be able to achieve sufficient brightness for area lighting at low voltages, the operating life of the OLED may be limited due to the heat generated by the high power level and relatively low efficiency of the device. To provide commercially viable light sources implementing OLEDs, the efficacy of the devices should be improved to reduce the heat generation when operating at a brightness sufficient to provide general illumination.
To emit light having a lumen output that is comparable to the light produced by conventional lighting sources such as fluorescent lighting sources, the OLED may be large, approximately one square meter, for example. A number of issues may arise when contemplating fabrication of a large OLED having a front surface area of one square meter, for instance. When fabricating OLED devices, conventional OLED devices implement top and bottom glass plates. Advantageously, glass substrates provide adequate hermeticity to seal the device from exposure to water and oxygen. Further, glass substrates allow for high temperature processing of the OLED devices. However, glass substrates may be impractical and less desirable when contemplating the fabrication of large area OLED devices for area lighting when compared to conventional area lighting sources, such as fluorescent lighting sources. Generally speaking, glass may be impractically heavy for area-lighting applications. For instance, to produce the light equivalent to a four foot T12 fluorescent lamp, for example, an OLED device implementing glass substrates having a thickness of ⅛ of an inch and a front surface area of one square meter may weigh approximately 31 pounds. The T12 fluorescent lamp weighs less than one-half a pound. One method of reducing the weight of the OLED device is to implement plastic substrates. However, while plastic substrates advantageously reduce the weight of the device, the hermeticity of the device may be compromised.
Further, as can be appreciated, general area lighting is widely used and the demands for such lighting are understandably high. Accordingly, to provide a viable alternative source for area lighting to that of fluorescent lighting, for example, the alternative source should be fairly robust and easy to manufacture. OLED devices implementing large glass substrates may be difficult to mass-produce in a highly automated process. The weight of glass and fragility of glass substrates may disadvantageously burden the manufacturing process.
Still further, as can be appreciated, the active layers of organic polymers or small-molecules implemented in OLED devices are disposed between conducting electrodes. The top electrode generally comprises a reflective metal such as aluminum, for example. The bottom electrode generally comprises a transparent conductive oxide (TCO) material, such as Indium-Tin-Oxide (ITO), that allows light produced by the active layers to be emitted through the bottom electrode. To maximize the amount of light that is emitted from the OLED device, the thickness of the ITO layer may be minimized. In typical OLED devices, the ITO layer has a thickness of approximately 1000 angstroms. However, the conductivity of 1000 angstroms of ITO may not be adequate to supply sufficient electrical current across the entire surface area of the large OLED. Accordingly, the electrical current may be insufficient to generate enough light across the large OLED for use in area lighting applications.
As can be appreciated, photovoltaic (PV) devices may be fabricated using similar materials and concepts as the OLED devices. Semiconductive PV devices are generally based on the separation of electron-hole pairs formed following the absorption of a photon from a light source, such as sunlight. An electric field is generally provided to facilitate the separation of the electrical charges. The electric field may arise from a Schottky contact where a built-in potential exists at a metal-semiconductor interface or from a p-n junction between p-type and n-type semiconducting materials. Such devices are commonly made from inorganic semiconductors, especially silicon, which can have monocrystalline, polycrystalline, or amorphous structure. Silicon is normally chosen because of its relatively high photon conversion efficiency. However, silicon technology has associated high costs and complex manufacturing processes, resulting in devices that are expensive in relation to the power they produce.
Like OLEDs, organic PV devices, which are based on active semiconducting organic materials, have recently attracted more interest as a result of advances made in organic semiconducting materials. These materials offer a promise of better efficiency that had not been achieved with earlier organic PV devices. Typically, the active component of an organic PV device comprises at least two layers of organic semiconducting materials disposed between two conductors or electrodes. At least one layer of organic semiconducting material is an electron acceptor, and at least one layer of organic material is an electron donor. An electron acceptor is a material that is capable of accepting electrons from another adjacent material due to a higher electron affinity of the electron acceptor. An electron donor is a material that is capable of accepting holes from an adjacent material due to a lower ionization potential of the electron donor. The absorption of photons in an organic photoconductive material results in the creation of bound electron-hole pairs, which must be dissociated before charge collection can take place. The separated electrons and holes travel through their respective acceptor (semiconducting material) to be collected at opposite electrodes.
While the particular layers of organic semiconducting materials that are implemented in PV devices, may differ from the particular layers of organic materials implemented in OLED devices, the similarity in structure between the PV devices and the OLED devices provides similar design and fabrication challenges. In some instances, techniques implemented in fabricating OLED devices may also be implemented in fabricating PV devices and visa versa. Accordingly, similar issues and challenges may arise in contemplating the fabrication of large area OLED devices and large area PV devices.